1. Field of the Invention
The present invention relates to a digital image processor, or in detail to the gradation control of a digital image forming apparatus such as a digital printer, a digital copying machine and the like.
2. Description of Related Art
In the electrophotographic process in a digital printer, a digital copying machine or the like, multi-level input image signals are obtained by reading a document and the light quantity of the laser beam for exposure is determined according to the image signals to form a latent image. The latent image is developed and transferred on a paper to reproduce the document image. In the image reproduction, it is desirable that the image density (output) after the image reproduction is proportional to the read image density signal (input) obtained by reading a document. The gradation characteristic which is a relation of the output image density against the read image density signal is an important factor which affects largely the impression of a pictorial image.
Then, the light quantity of the laser beam is corrected according to the input density signal to realize the above-mentioned linear relation between the input read data and the output image density. This is called as the gamma correction.
The gradation characteristic changes subtly with the photoconductor sensitivity, the surface potential, the development bias voltage, the development characteristic and the like. Then, the image reproduction is stabilized by using automatic image density control, the gradation correction and the like to realize the linear characteristic.
In a digital image forming apparatus, the read density of a document is converted to multi-level digital data. Because nonlinear conversion of multi-level data is easy by using a look-up table or the like, various types of stabilization control and the gradation characteristic are possible for a digital image forming apparatus. (Refer, for example, to Japanese patent laid open Publication No. 271,764/1991.)
However, the image stabilization is not perfect actually, and the quality of the reproduced image with use of the image stabilization is not satisfactory for some experienced users. Further, if a user can select the gradation characteristic in a wide range, a user can reproduce an image of his or her taste. Then, the inventors consider that the user will be satisfied if he or she can change the gradation characteristic positively.
In order to make it possible for a user to change the gradation characteristic, the gradation correction and the image stabilization have to be operated simultaneously, and a process controller is required suitable for each image reproduction process.
When a user is allowed to change gradation characteristic, if the number of gradation characteristics to be selected increases, the memory capacity for the gradation characteristic data increases to a large extent and this is problem to be solved.